The invention relates to an improvement in low specific speed centrifugal pumps.
It is well known that frictional and mixing losses limit the efficiency of centrifugal pumps and that such losses decrease and increase, respectively, as specific speed increases. Specific speed is a design characteristic number designated as Ns and equals ##EQU1## wherein .OMEGA. is flow rate in gallons per minute, rpm is pump impeller revolutions per minute and H is the developed head of a pump in feet. For pumps having relatively low specific speeds, i.e., below the range of about 600-700, frictional losses exceed mixing losses and have a substantial effect on pump performance.
Low specific speed pumps provided with cast metal casings are known to be subject to substantial variations from design pump performance, due to substantial variations in frictional losses resulting from minor variations in size of the pump chamber and the surface finish thereof commonly encountered with metal castings. While it is possible to machine or hand work the internal surfaces of such pumps, with a view towards obtaining greater uniformity in size and surface finish in order to decrease the overall variations in efficiency, to do so adds substantially to manufacturing costs.